In your method, add the new printers to the arraylist.
while (result.next()) {
Printer printer = new Printer();
printer.setPrinterid(result.getString("printerid"));
printer.setNaam(result.getString("printernaam"));
printer.setAantalKl(result.getInt("aantalkleur"));
printer.setAantalZw(result.getInt("aantalzwart"));
printers.add(printer); // Adds the printer to the arraylist.
}
Then print it.
System.out.println(selectAllePrinters());